1. Field of the Invention
The present invention relates to a data transmission system in a cyclic digital transmission used in a monitoring station.
2. Description of the Related Art
FIG. 1 shows the structure of a monitoring system that communicates alarm data by means of this type of cyclical data transmission (hereinbelow abbreviated CDT) of the prior art. In FIG. 1, the monitoring system is made up of monitoring device 10 established at master station 1, monitoring device 20 established at slave station 2, and the circuit connecting the monitoring devices.
An alarm from a monitored device that is collected within slave station 2 is converted to message data at external interface 23 of monitoring device 20 and sent to table comparison section 22. Table comparison section 22 compares message data sent from external interface 23 with CDT conversion table 24 and determines the word position in the assembly destination of alarm data.
In CDT conversion table 24, message data are pre-defined by each word position in an assembly destination of alarm data. FIG. 2 shows the format of the CDT conversion table. The CDT conversion table indicates by the numerical values of words and bits the CDT output positions in an assembly destination corresponding to message data of an alarm, and holds the combinations of all words and bits.
Monitoring device 20 then transmits alarm data of the determined word positions from CDT transmitter 21 to CDT receiver 11 of monitoring device 10 established at master station 1. Fundamentally, the total number of words of a one-cycle portion (for example, 64 words, one word being made up of 13 bits) of alarm data are always outputted, but the number of output words can be altered by using output word setting section 25 to pre-set the number of words in one cycle.
This transmission of information between a master station and slave station by means of this type of CDT is always carried out cyclically, and assembly for CDT is carried out by basing the assembly of CDT output words on a fixed CDT conversion table in which assembly words are pre-defined for each item of message data of an alarm, and by always referring to the entire table for all words (64 words). The conversion process is therefore time-consuming, and as a result, there is the problem that recognition of the generation of an alarm is delayed at the master station, i.e., the host center monitoring device.
In monitoring systems of the prior art, the table must be rewritten for all words on the transmitting side and receiving side in cases of the addition or deletion of data of a table designating assembly words, such addition or deletion commonly accompanying increases or decreases in collected alarms. In monitoring systems of the prior art, moreover, in cases in which there are fewer categories of collected alarms than the total number of CDT output words per circuit, words are outputted for which no definition exists, resulting in the transmission of superfluous words. Although the number of output words can be altered by means of output word setting section 25, this modification must be effected in advance.